Legionnaires' disease

Legionnaires’ disease, caused by Legionella bacteria, can result in pneumonia and infections of several body parts. Isolated cases and outbreaks occur in all countries in Europe, with the majority acquired outside of healthcare institutes. Since 2017, around 10-20% of the annually reported cases are travel related (ECDC, 2012-2023). While the disease is an uncommon and mainly sporadic respiratory infection in Europe, it is substantially underdiagnosed and underreported (ECDC, 2012-2023). Increasing temperatures, higher rainfall intensity and more extreme events may affect bacterial growth and usage of water (where the bacteria is present), which may increase the disease incidence in certain regions.

Source & transmission

Of the different Legionella species that exist, L. pneumophila is responsible for most of the Legionella infections in people, mostly affecting the lower respiratory tract (Kozak-Muiznieks et al., 2018). Legionella bacteria are common and are present in low numbers in natural rivers and lakes. In artificial aquatic systems like cooling towers, reservoirs, evaporative condensers, humidifiers, decorative fountains, hot water and similar systems, the bacteria can easily multiply and pose a health risk.

Humans are mainly infected by inhalation of aerosols, i.e., water droplets in the air that contain Legionella bacteria. Inhalation of water droplets in the air surrounding surgical wounds, or direct contact with the wound, could also infect people (Kashif et al., 2017). Drinking contaminated water poses no risk and also infections via person-to-person contact are extremely rare (Correia et al., 2016).

Legionnaires' disease outbreaks in holiday accommodations are mostly associated with water systems of hot or cold water, including water tanks, hotel room outlets, showers located in swimming pools and spas, or garden sprinklers. If bacteria concentrations in the water are high, a person inhaling the bacteria trapped in the water droplets can be infected whilst taking a shower or bath (Papadakis et al., 2021). Large outbreaks are often associated with cooling towers or so-called wet-air conditioning systems. When the Legionella bacteria are present in such systems, they can quickly multiply and pose a risk in public facilities that use water for air conditioning such as hotels. Dry air conditioning systems are not dangerous.

Legionnaires’ disease can also be acquired in hospitals when Legionella bacteria colonize the water systems and cause infections via bathing, steam-heated towels, humidifiers, decorative fountains, and certain medical devices (Beauté et al., 2020).


Health effects

Legionnaire's disease usually starts with a dry cough, fever, headache and sometimes diarrhoea. Infections with Legionella bacteria often result in a form of pneumonia several days after infection. Most commonly, lungs and the gastrointestinal tract are affected. In severe cases, Legionnaires' disease may affect several organs and body parts, leading to high mortality rates. Due to similar symptoms, Legionnaires' disease is often misdiagnosed as a regular pulmonary infection. However, diarrhoea and the presence of specific enzymes in the blood, can indicate a Legionella bacteria infection. When several people are diagnosed at once, this may indicate an outbreak and a common source of infection may be identified.

Morbidity & mortality

In the EEA member countries (excluding Switzerland and Türkiye due to absence of data), in the period 2005-2022:

  • 117,605 infections (ECDC, 2023)
  • The highest annual notification rate to date in the EU/EEA was observed in 2021 with 2.4 cases per 100 000 population.
  • The fatality rates vary between 7 and 9%.
  • An increasing number of cases have been reported between 2014 and 2022, except in 2020 during the COVID-19 pandemic due to underreporting and decreased exposure.
  • Travel-related cases amounted to 15-20% before the pandemic, but fell down to below/around 10% in 2020-2021, at least partly due to the pandemic and associated travel restrictions.

(ECDC, 2012-2023)

Distribution across population

  • Age group with the highest disease incidence in Europe: > 65 years old, with more than 90% of all cases being reported in people over 45 years old (ECDC, 2012-2023)
  • Groups at risk of severe disease course: people over 45 years, smokers, people with low immunity or a poor health status


Climate sensitivity

Climatic Suitability

Legionella bacteria are known to have a wide temperature tolerance, being able to withstand temperatures between 0 and 68°C and to grow between 25 and 42°C with the fastest growth at 35°C (Spagnolo et al., 2013).


In Europe, most infections occur between June and October, with peaks in the summer months when temperatures are higher in some years (ECDC, 2012-2023).

Climate Change Impact

Legionnaires' disease can increase with increasing annual precipitation and mean temperature, rainfall intensity and duration, associated to climate change (Han, 2021; Pampaka et al., 2022). Increasing precipitation amounts are the most important driving climatic factor since Legionella bacteria are water-borne. More frequent or intense drought periods induce low flow rates, which on the other hand can also increase bacterial growth. In addition, increasing air temperatures favour bacterial growth in most European countries, since optimal conditions for bacterial growth are not too often passed, e.g.,  optimal growth happens at 35°C for Legionella bacteria (Spagnolo et al., 2013). Due to the shift in temperature and rainfall conditions, which are becoming more suitable for Legionella, a northward expansion of the bacteria and the associated disease is likely in Europe and previously unaffected areas may experience Legionnaires’ disease cases or outbreaks.


Prevention & Treatment


  • Proper maintenance of artificial water systems and avoidance of risk factors (including organic material, warm water temperatures (25-42°C) and low flow rates), e.g., by circulating hot water (> 60°C)
  • Maintenance of water supply systems for drinking water and decorative fountains that may spread aerosols and droplets, e.g., by supplying water at temperatures below 25 °C and cleaning regularly
  • Maintenance of safe water environments for recreational activities by using frequent cleaning and the use of disinfectants (e.g., chlorine)
  • Improved water safety management on ships, e.g., by maintaining water temperatures outside of the favourable range for Legionella bacteria and disinfecting regularly
  • Legionaires’ disease surveillance, e.g., the European Legionnaires’ Disease Surveillance Network (ELDSNet), to allow for disease detection and subsequent response measures to prevent spreading of the disease

(National Academies of Sciences, Engineering, and Medicine, 2020; Sciuto et al., 2021)




Links to further information



Beauté, J., et al., 2020, Healthcare-Associated Legionnaires' Disease, Europe, 2008-2017, Emerging Infectious Diseases 26(10), 2309-2318. https://doi.org/10.3201/eid2610.181889 

Correia, A.M., et al., 2016, Probable Person-to-Person Transmission of Legionnaires’ Disease, New England Journal of Medicine 374 (5), 497-498. https://10.1056/NEJMc1505356 

ECDC, 2012-2023, Annual epidemiological reports for 2010-2021 – Legionnaires’ disease. Available at https://www.ecdc.europa.eu/en/legionnaires-disease/surveillance-and-disease-data/surveillance. Last accessed August 2023.

ECDC, 2023, Surveillance Atlas of Infectious Diseases. Available at https://atlas.ecdc.europa.eu/public/index.aspx. Last accessed August 2023.

Han, X. Y., 2021, Effects of climate changes and road exposure on the rapidly rising legionellosis incidence rates in the United States, PLOS ONE 16(4), e0250364. https://doi.org/10.1371/journal.pone.0250364

Kashif, M., et al., 2017, Legionella pneumonia associated with severe acute respiratory distress syndrome and diffuse alveolar hemorrhage—A rare association, Respiratory Medicine Case Reports 21, 7–11. https://doi.org/10.1016/j.rmcr.2017.03.008

Kozak-Muiznieks, N. A., et al., 2018, Comparative genome analysis reveals a complex population structure of Legionella pneumophila subspecies, Infection, Genetics and Evolution 59, 172–185. https://doi.org/10.1016/j.meegid.2018.02.008

National Academies of Sciences, Engineering, and Medicine, 2020,  Management of Legionella in Water Systems. Washington, DC, The National Academies Press. https://doi.org/10.17226/25474

Pampaka, D., et al., 2022, Meteorological conditions and Legionnaires’ disease sporadic cases-a systematic review, Environmental Research 214, 114080. https://doi.org/10.1016/j.envres.2022.114080

Papadakis, A., et al., 2021, Legionella spp. Colonization in Water Systems of Hotels Linked with Travel-Associated Legionnaires’ Disease, Water 13(16), 2243. https://doi.org/10.3390/w13162243 

Sciuto, E. L., et al., 2021, Environmental Management of Legionella in Domestic Water Systems: Consolidated and Innovative Approaches for Disinfection Methods and Risk Assessment, Microorganisms 9(3), 577. https://doi.org/10.3390/microorganisms9030577

Spagnolo, A. M., et al., 2013, Legionella pneumophila in healthcare facilities, Reviews in Medical Microbiology 24(3), 70–80. https://doi.org/10.1097/MRM.0b013e328362fe66